Stocked and wild trout diet was assessed in two north-eastern Portugal headstreams during the summer season of three successive years (2000 to 2002). Significant differences were detected in the diet composition between stocked (age 1+) trout and distinct size-class of wild trout. Stocked fish showed preference, almost exclusively, for food items captured near the surface (primarily terrestrial adult insects), emergent pupae and subimagos. In contrast, young-of-year (YOY) wild trout fed predominantly on the most available aquatic prey taxa such as Chironomidae (Diptera) larvae and Baetidae (Ephemeroptera) nymphs; however, Trichoptera larvae were not taken. Wild trout preference changed from benthic aquatic invertebrates to terrestrial origin organisms and this ratio increased with size, suggesting an ontogenetic diet variation. Significant diet overlap was only detected between stocked and dominant wild trout, which were not displaced from the energetically profitable areas. Therefore, this study showed the absence of an evident competition for food in both streams, even when trout density was largely augmented by stocking activity. However, since no obvious benefits on space and time were obtained, stocking must be carefully evaluated and alternative approaches considered, like habitat improvement and adequate fisheries management, in order to increase the natural productivity of these systems.
Fish assemblage in the Úpoř brook was observed. The stream was inhabited by 13 fish species and Lampetra planeri. The abundance reached the mean value of 12175 spec.ha-1 and the mean biomass 395 kg.ha-1. Salmo trutta dominated, Cottus gobio was also very abundant and inhabited preferably riffles as well as Barbatula barbatula. Leuciscus cephalus and L. leuciscus were resident in pools. Other species occurred occasionally. Preferences in habitat and microhabitat use were evaluated using RDA and FE test. Two first axes of RDA explain 27.8% of total variability of all species, and 95.3% of relationship between the species and the environmental variables. These factors significantly correlated with the axes: distance from the mouth, maximum depth, presence of mud, presence of stones as shelters, presence of rocky shelters, and presence of the deposits. The character of substrates was the most important factor influencing the distribution of S. trutta, L. cephalus, L. leuciscus and B. barbatula, whereas C. gobio were greatly influenced by character of shelters. Also the distance from the mouth play a role in distribution of two last species. Seasonal changes in habitat preference of C. gobio were recorded.
The level of introgression between wild and stocked individuals in Portuguese brown trout Salmo trutta L. populations was evaluated. Fish were sampled from rivers of the Douro Basin and compared with fish used in restocking programme. This study, based on microsatellite and allozyme loci, showed, thanks to the most discriminant loci, a low percentage of heterozygote individuals. This happened due to absence or low levels of intercrossing between stocked and native brown trouts. It was confirmed the inefficiency of stocking practices in Portugal, as suggested by mtDNA haplotype distribution. From the management and conservation points of view, any kind of stocking with exogenous specimens should be avoided to allow preservation of the autochthonous genetic pools. As well as the stocking techniques and released fish density should be considered. Therefore new policies for stocking and monitoring hatchery fish are needed to preserve the gene pools of wild Portuguese trout populations.
In the years 1999-2002, first studies were carried out on the metazoan parasites of Atlantic salmon (Salmo salar L.) recently reintroduced into the Elbe River drainage system, after more than 50 years after the complete extinction of the Elbe salmon population. A total of six helminth species were recorded from salmon smolts from three streams of the Elbe River basin (Kamenice River, Ještědský and Libočanský Brooks) in North Bohemia, Czech Republic, where S. salar fingerlings have been released since 1998: Gyrodactylus truttae Gläser, 1974, Crepidostomum metoecus Braun, 1900, Diplostomum spathaceum (Rudolphi, 1819) metacercariae, Raphidascaris acus (Bloch, 1799) adults and encapsulated larvae, Cystidicoloides ephemeridarum (Linstow, 1872), and Neoechinorhynchus rutili (Müller, 1780). Except for D. spathaceum, all these freshwater parasites have been received from the helminth fauna of the co-habiting brown trout (Salmo trutta fario L.). Due to local ecological conditions, the parasite faunae of both salmon and brown trout exhibited distinct qualitative and quantitative differences in the three localities. The finding of G. truttae on S. salar represents a new host record. Three helminth parasites of marine origin, the cestodes Eubothrium crassum (Bloch, 1779) and Scolex pleuronectis Müller, 1788 plerocercoids, and the nematode Anisakis simplex (Rudolphi, 1809) larvae were found in the single examined adult S. salar from the Kamenice River. New data on the geographical distribution of some nematode parasites of Salmo trutta fario L., Barbatula barbatula (L.) and Anguilla anguilla (L.) are presented.
Proliferative kidney disease (PKD) is a widespread temperature-dependent disease in salmonids caused by the myxozoan parasite, Tetracapsuloides bryosalmonae (Canning, Curry, Feist, Longshaw et Okamura, 1999) (Tb). Tb has a two-host life cycle, involving fish as an intermediate host and freshwater bryozoans as the definitive host. Although salmonids are acknowledged as hosts for the parasite, it is less clear which fish species are active hosts in the life cycle of Tb. Differences in infection dynamics have been observed between some fish species, which are thought to be related to the existence of two main Tb-strains, the American and European. Iceland, having three species of indigenous salmonids and positioned geographically between Europe and North America, is an ideal location to study the natural development of Tb in wild fish. The main aim of this study was to determine the genetic origin of Tb in Iceland and confirm whether mature spores are produced in Icelandic salmonids. In this study, Icelandic salmonids were infected with the European Tb-strain. In situ hybridisation revealed that intraluminal sporogonic stages, including mature spores, were commonly observed in all three salmonid species. The presence of intraluminal stages has previously been confirmed in brown trout Salmo trutta Linnaeus and Atlantic salmon S. salar Linnaeus in Europe, but they have only been observed in Arctic charr Salvelinus alpinus (Linnaeus) in North America, infected by the local strain. This is, therefore, the first time that sporogonic stages have been observed in Arctic charr in Europe, where fish are infected with the European Tb-strain. Our data strongly suggest that all the three salmonid species inhabiting Icelandic waters serve as active hosts in the life cycle of Tb. However, for full confirmation, transmission trials are needed.